1. Field of the Invention
This invention relates to an image drawing apparatus which draws an image in accordance with image data, and more particularly to an image drawing apparatus suitable for use with, for example, a laser beam printer, a copying machine and like apparatus.
2. Description of the Related Art
Conventional laser beam printers are normally constructed such that a single laser beam is scanned at a high speed on a rotary drum by means of a polygon mirror while the rotary drum is rotated at a predetermined speed to draw a predetermined image on the rotary drum. Since it is difficult to set all faces of the polygon mirror in parallel to an axis of rotation thereof, inclination of the faces of the polygon mirror with respect to the axis of rotation is corrected using a correcting lens. Since the distances from the source of the laser beam to a central position and an end position are different from each other, deformation of an image takes place. In order to correct the deformation of the image, an f.theta. lens is interposed between the polygon mirror and the rotary drum. Consequently, spool-shaped deformation of an image which takes place where no f.theta. lens is interposed is corrected.
In order to achieve high speed printing with such a printer, it is necessary to raise the speed of rotation of the polygon mirror to a high speed. For example, in order to print the size A4 in one second with 400 DPI, the speed of rotation of the polygon mirror which has six faces is about 50,000 rpm. Further, in this instance, when the gradation of half tones includes 256 steps, the laser modulating frequency is about 4 GHz.
The conventional laser beam printer has a limitation in increase of the printing speed since it achieves high speed printing by rotating the polygon mirror at a high speed in this manner. Further, there is a subject that not only high noise is generated by high speed rotation of the polygon mirror, but also the life of the laser beam printer is shortened. In addition, since an f.theta. lens having a high degree of accuracy is expensive, also the entire laser beam printer is expensive.